1. Field of the Invention
The present invention belongs to the field of steering and drive control systems used in electric, four-wheel-drive and four-wheel steering vehicles that have on-board battery power supplies, and that are operated at low speeds in various indoor and outdoor facilities other than public streets and highways, for example, hospitals and nursing homes, libraries, amusement parks, theme parks, sports and entertainment centers, offices, supply distribution bases, computer warehouses, large commercial facilities, etc. More specifically, the present invention is related to an electric vehicle steering/drive control method, a steering/drive system therefor, and an electric vehicle in which the steering and drive are controlled by this steering/drive system and method, wherein a variety of steering modes are provided and the driver selects from among these modes, the steering mode that is most appropriate for the instant driving conditions.
2. Description of Related Art
The elderly now constitute an ever-growing segment of society, and are using hospitals and nursing homes to an unprecedented extent. The number of wheelchair-bound individuals who are either hospital patients, or who use a variety of other facilities, also continues to grow. Along with this aging cross-section of society, medical treatment facilities and nursing facilities are tending to be much larger in size than they used to be. In addition, in the future, the elderly and handicapped are expected to play a more active role in society.
When considering the situation described above, one can reasonably predict a need for a variety of electric vehicles for supplementary transportation, not only for the elderly and handicapped, but also for other users of a wide variety of indoor and outdoor facilities that lack roadways. These facilities include hospitals, nursing homes, ordinary commercial and industrial facilities including supply distribution bases, computer warehouses, large commercial buildings, libraries, amusement parks, sports and entertainment facilities, and office buildings. Such transportation would be provided as a service to users of the facilities, and would also be provided to employees to improve efficiency in the day-to-day operation of the facilities. One can further reasonably predict that the future will bring a substantial need for various kinds of electric vehicles for moving packages, merchandise, etc., around various facilities at low speeds.
The vehicle passageways in such facilities are subject to a variety of constraints that are unique to each facility. Some of these constraints strongly influence requirements for steering and straight-running operation of vehicles within the facility, e.g., constraints on the width, area and shape of vehicle passageways in lobbies, waiting rooms, hallways and elevators; and the placement of structural objects such as pillars, walls and bordering structures at the edges of passageways. Therefore, a special need exists for vehicles that are used in such facilities that have steering capabilities that enable them to function well and move quickly to a given location and maneuver easily into a desired position under conditions imposed by the passageway constraints of the respective facility.
Passageways in such facilities are normally not wide enough, nor do they have enough driving area at intersections, to give conventional vehicles adequate room to run straight, pass, and turn left and right without difficulty. Also, with the limited amount of floor space available in cramped elevators, lobbies and waiting rooms, for vehicles operating under the severe passageway constraints of these spaces, there is a need for means for enabling a vehicle to quickly turn, change direction and move sideways to place itself in a desired position without striking any surrounding objects.
A survey of conventional electric cars, however, will reveal that none of them have the maneuverability to operate in this manner within the passageway constraints of the various types of facilities. There are indeed four-wheel-drive electric cars that are commonly known and offered for practical use. The steering mechanisms of all of these cars, however, use either Ackerman-type link mechanisms made up of knuckle arms, tie rods, etc., or mechanisms that depend on a mechanical structure using differential gears. Also, when steered into a turn, all of these conventional vehicles, have a wheel path pattern referred to as an “inside wheel difference” pattern, i.e., in the wheel travel path pattern formed by a vehicle turning left or right, the rear wheels fall to the inside of the arc traveled by the front wheels. Accordingly, if one of these conventional electric cars in which the wheels always inscribe an inside wheel difference pattern were to be put into use—not on an ordinary street, but in one of these facilities in which passageway constraints are imposed—the vehicle would be found incapable of easily turning, changing its direction, or moving sideways.
An object of the present invention, therefore, is to provide a four-wheel-drive electric vehicle steering/drive control whereby a number of different wheel travel path patterns can be realized to enable the vehicle to negotiate turns, changes in direction, and sideway movements swiftly and accurately under a variety of facility-specific passageway constraints to smoothly and quickly maneuver the vehicle into a desired position.
Another object of the present invention is to provide a steering control means wherein all steering and drive control functions can be accomplished entirely by electrical means, using no mechanical systems Ackerman-type link mechanisms, differential gears etc.; wherein a variety of steering control functions can be performed; and wherein the configuration of the steering control system is simple.
In such a system, the travel paths traced by the four wheels when the steering wheel is turned are not all the same. Therefore the steering angles and speeds-of-rotation of the wheels must be controlled according to the arc lengths of the travel paths of the respective wheels. Moreover, unless this control is exact, it can result in skidding or wheel-spinning during turns, and/or “toe-in” and “toe-out” when running straight. Toe-in is a condition in which the front edges of left and right wheels are closer than their rear edges; and toe-out is the opposite condition, wherein the rear edges are closer. Either condition can impede or prevent the forward travel of the vehicle. In the present invention such undesirable phenomena are prevented by providing theoretically exact control of steering angle and rotation speed.
In addition, in the present invention, the control system is designed so that no significant imbalance of drive power applied to the respective wheels will occur as a result of changes in effective tire diameter due to tire wear, changes in tire pressure, or changes in the load applied to the tire; and so that smooth steering and drive control will continue undisturbed even in the presence of abrupt changes in the vehicle speed and direction commands executed by the driver.